Pin SDK to preview 8 in source build (#13707)

- There are issues with NU5048 and NU5128. We need to handle ProjectIconUrl deprecation and issues with NuspecProperty not being set separately.

Author: John Luo

.azure/pipelines/ci.yml

@@ -510,7 +510,9 @@ jobs:
       # The SDK version selected here is intentionally supposed to use the latest release
       # For the purpose of building Linux distros, we can't depend on features of the SDK
       # which may not exist in pre-built versions of the SDK
-      version: 3.0.x
+      # Pinning to preview 8 since preview 9 has breaking changes
+      # version: 3.0.x
+      version: 3.0.100-preview8-013656
       installationPath: $(DotNetCoreSdkDir)
       includePreviewVersions: true
   - script: ./eng/scripts/ci-source-build.sh --ci --configuration Release /p:BuildManaged=true /p:BuildNodeJs=false